The present invention relates to a developing apparatus usable with an image forming apparatus wherein a multi-color image or a full-color image is formed by an electrophotographic process, an electrostatic recording process, a magnetic recording process or the like.
In the field of electrophotography, many proposals have been made to form multi-color, e.g., two color image or a full-color image, wherein the apparatus is required to have a plurality of developing devices. In order to provide a full-color image, in a copying apparatus, for example, one developing device is required for each of yellow toner, magenta toner, cyan toner and black toner, if required, so as to develop a latent image corresponding to each of the color-separated images. As for a method of arranging the developing devices around a photosensitive member, they are all located around the photosensitive member along its surface. Further, Japanese Patent Application Publication No. 20579/1980 filed by the assignee of this application proposes that a plurality of developing devices are fixedly supported on a revolvable supporting turret so that the developing openings of the developing devices are selectively opposed to the surface of the photosensitive member by revolving the turret. The latter is advantageous in that only one developing device is opposed to the photosensitive member with the result that the circumferential length of the photosensitive member measured along the direction of its movement can be reduced so as to reduce the size of the whole apparatus.
Generally, the developing device is required to have detecting means therein in order to detect the amount of the toner therewithin and the mixture ratio of the toner and carrier. However, the detection may be erroneous when the toner is attached to the sensing surface of the detecting means. It is necessary to provide a member for cleaning the sensing surface of the detecting means to remove the toner therefrom. When such a member is used, a driving mechanism for the cleaning member is required. However, in the device described above wherein the plurality of the developing devices are supported on a revolvable turret so as to oppose a required one of them to the photosensitive member, the cleaning member and the associated driving mechanism results in a much complicated structure.